1. Field of the Invention
The invention relates to a braking system for motor vehicles having                a master cylinder to which wheel brake circuits are connected,        a first piston which is coupled to a brake pedal via a pressure rod which transmits actuating forces,        a second piston by means of which the master cylinder is actuated,        a third piston which can be actuated by the first piston and which can be connected in a force transmitting fashion to the second piston,        having a simulation device with at least one elastic element which gives the driver a pleasant pedal sensation,        having an intermediate space, to which hydraulic pressure can be applied, between the second piston and third piston, wherein application of pressure to the intermediate space loads the second and third pistons in opposite directions,        having a hydraulic chamber which is bounded by the third piston, can be disconnected by means of a check valve and with which, when necessary, a movement of the third piston in the actuating direction is prevented,        having a pressure generating device which controls the pressure in the intermediate space,        having a pressure medium reservoir vessel which is under atmospheric pressure and which can be connected hydraulically to the intermediate space,        and having means for electrically controlling the pressure applied to the intermediate space.        
2. Description of the Related Art
Brake-by-wire braking systems are becoming increasingly widespread in motor vehicle engineering. In these braking systems, the brake can, on the one hand, be actuated “externally” without active intervention by the driver of the vehicle on the basis of electronic signals. These electronic signals can be output, for example, by an electronic stability program ESP or a distance control system ACC. Otherwise, it is possible to dispense entirely or partially with actuation of the braking system if a braking effect which is requested by the driver of a vehicle by actuation of a brake pedal is brought about, for example, by switching over an electric vehicle drive into a generator mode. In both cases, the actuating state of the brake does not correspond to the actuation of the brake pedal which is predefined by the driver of the vehicle. In conventional braking systems, this leads to a reaction on the brake pedal. The brake pedal characteristic, i.e. the dependence of the brake pedal travel on the brake pedal force, is disrupted by the described reaction. This reaction effect on the brake pedal can be surprising and unpleasant for the driver, with the result that in a critical situation of road traffic the driver does not actuate the brake pedal to a degree which is adapted to this situation since he is irritated by the reaction on the brake pedal which he cannot predict.
A braking system of the generic type mentioned at the beginning is known from DE 10 321 721 A1. The braking system which is disclosed in the aforesaid publication can be used, inter alia, in vehicles with a hybrid drive in which what are referred to as regenerative braking processes are carried out. The pressure control valve which serves to control the pressure applied to the abovementioned intermediate space is driven using mechanical force transmitting means in the previously mentioned braking system, which means are arranged effectively between the first piston and the valve body of the pressure control valve. Electromagnetically drivable valve devices, which are embodied as (currentless closed) 2/2 way valves which can be controlled in an analog fashion and closed in the currentless state, are provided as means for electrically controlling the pressure applied to the intermediate space. It is considered disadvantageous with the previously known braking system that the actuating energy which is necessary to operate it has to be kept available in a high pressure accumulator whose pressure medium flows into the wheel brake circuits during actuation of the brakes. On the one hand, it is energetically inefficient firstly to raise the pressure medium to the high pressure level of the high pressure accumulator even though only a fraction of this accumulator pressure level is required for the majority of braking operations, and on the other hand this entails the risk that in the event of possible contamination of the pressure medium by gas bubbles, the braking system can fail as a result of the fact that the volume expansion of the gas bubbles which is associated with the flow out of the high pressure accumulator said gas bubbles can expel from the braking lines a column of liquid which is necessary for the hydraulic transmission of force.